This invention relates to improved electrical circuit apparatus, and a method of manufacturing same, and more particularly, to such apparatus which is relatively inexpensive to manufacture and is capable of performing different circuit operations which, heretofore, were carried out by separate circuits mounted on separate circuit boards.
Although integrated circuit technology is used to manufacture self-contained circuits which are capable of operating independently, without requiring substantial interconnections with other circuitry, many circuit applications still require that integrated circuits and discrete components be coupled to each other in order to perform desired circuit functions in various different types of devices. In general, different circuit functions may be performed by circuitry mounted on separate circuit boards, commonly referred to as printed circuit boards. For example, in a radio receiver, various ones of these printed circuit boards are provided, some having radio tuning circuitry thereon, and others having pre-amplifiers, power amplifiers, and the like. One of ordinary skill in the art will appreciate that other devices likewise may be provided with various printed circuit boards having different circuit elements mounted thereon for carrying out particular circuit functions.
In an attempt to minimize the manufacturing costs of such devices, it has been proposed that the various circuit boards which are used be of substantially the same size. Likewise, to facilitate assembly, it has been proposed that as many of the circuit elements as possible be of the same form and shape. Thus, irrespective of the particular functions to be performed thereby, the circuit elements and circuit boards upon which such elements are to be mounted are attempted to be standardized. Many of such standardized circuit elements are provided with relatively small, or even no, conducting leads. Hence, other connections, such as solder, are needed to interconnect the electrodes of such circuit elements to the conductive patterns which generally are provided on at least one surface of the circuit boards.
A typical circuit board which is used to mount circuit element thereon is formed of, for example, a phenolic resin. A pattern of conductive leads is provided on at least one surface of this circuit board by depositing a copper leaf thereon and etching that leaf so as to form the desired conductive pattern. One advantage of using a phenolic resin board is that such boards are relatively inexpensive. Hence, the overall cost of the circuitry which is formed by mounted circuit elements on a phenolic resin board likewise is relatively inexpensive. Another advantage is that such phenolic resin circuit boards have a relatively low thermal radiation coefficient, that is, a low coefficient of thermal diffusion. Because of this, repairs to, or replacement of, circuit elements mounted on such a phenolic resin board may be achieved by using a soldering iron. Also, these boards may be processed readily, that is, they may be cut, shaped and otherwise manipulated, as desired.
However, if an exothermic circuit element is mounted on a phenolic resin board, that is, a circuit element which emits substantial heat, that heat can be dispersed best by providing a heat sink on the board. This is because the low coefficient of thermal diffusion of the phenolic resin board does not facilitate dispersal of the heat. This adds to the overall labor, and thus cost, of manufacturing circuits in which exothermic circuit elements are used, such as power amplifiers.
Another disadvantage of phenolic resin boards is that such boards have measurable dielectric constants. Because of such dielectric constants, the phenolic resin boards influence the response characteristics of circuits mounted thereon that are used in higher frequency ranges, such as very high frequency (VHF) circuits, ultra-high frequency (UHF) circuits, super high frequency (SHF) circuits and extremely high frequency (EHF) circuits. For example, the phenolic resin circuit board may deleteriously affect the signal-to-noise (S/N) ratio as well as the high impedance of such circuits. Furthermore, surface leakage of the phenolic resin board may present a problem to the satisfactory performance of these circuits.
Accordingly, where circuitry is mounted on a circuit board for use in the higher frequency ranges, other materials, such as glass epoxy resin, should be used. However, with glass epoxy resin, a high performance ink for a solder mask frequently must be coated thereon. The use of a glass epoxy resin is relatively expensive and additional processing steps are needed when the glass epoxy resin board is used to support circuit elements. Furthermore, large areas of a glass epoxy resin board must be printed with conductive ink so that a preferable distribution of circuit elements may be mounted, thereby avoiding localized concentrations. In place of a glass epoxy resin, it has been proposed to use alumina ceramic boards. However, such alumina ceramic circuit boards suffer from many of the same disadvantages noted above with respect to glass epoxy resin boards.
When the circuit board is to have exothermic circuit elements mounted therein, neither phenolic resin nor glass epoxy resin are optimum materials. Rather, aluminum or alumina ceramic circuit boards have been used. However, these types of circuit boards are relatively expensive to manufacture and, moreover, because of their favorable coefficients of thermal diffusion, it is relatively difficult to repair such circuit boards by using a soldering iron.
Therefore, it is believed that there is a definite need for improved circuit apparatus, including circuit boards which are readily adapted to have different types of circuit elements mounted thereon, while being free of the defects and disadvantages discussed above.